1. Field of the Invention
This invention relates to a power transmission system for vehicles, mainly for motorcycles.
2. Description of the Prior Art
When a power transmission system for vehicles receives an excessively large negative lead due to a sudden deceleration of the engine, it is generally vibrated. A power transmission system for vehicles, which is designed with a view to minimizing such vibrations by preventing an excessively large negative load from being transmitted to the engine, has already been proposed. In this power transmission system, a driving member connected to a crankshaft of the engine and a driven member connected to a driving wheel are joined together via a one-way clutch and a friction clutch, which are disposed parallel to each other, the one-way clutch being so formed that it is put in a connected state when a positive load is applied thereto and in a disconnected state when a negative load is applied thereto, the friction clutch being so formed that it is slided when a rotary torque at a level not lower than a predetermined level is applied thereto. According to this power transmission system, when a positive load is applied to the engine, the power of the engine can be transmitted without slip to the driving wheel via the one-way clutch. On the other hand, when a negative load is applied to the engine, the friction clutch is slided, so that transmission of a negative load at a level not lower than a predetermined level can be prevented. However, the above-described power transmission system has the following drawbacks. Namely, when an engine is so-called push-started, i.e. when a vehicle is moved by being pushed by the human power to apply the driving torque to the driving wheel and crank the engine with the driving torque of a starting motor interposed between the driven member and driving wheel, the driving torque is partially absorbed into the friction clutch. Thus, the driving torque cannot be transmitted to the engine efficiently. This would cause the starting capability of the engine to be lost.